OPML support (and ability to add custom attributes)

[sjehuda]

OPML support would be useful. Adding custom attributes would be good too:

• Custom filter rules (See Liferea).
• Custom tags (See Slixfeed).

  def export_to_opml(jid, filename, results):
  print(jid, filename, results)
  function_name = sys._getframe().f_code.co_name
  logger.debug('{} jid: {} filename: {}'
              .format(function_name, jid, filename))
  root = ETR.Element("opml")
  root.set("version", "1.0")
  head = ETR.SubElement(root, "head")
  ETR.SubElement(head, "title").text = "{}".format(jid)
  ETR.SubElement(head, "description").text = (
      "Set of subscriptions exported by Slixfeed")
  ETR.SubElement(head, "generator").text = "Slixfeed"
  ETR.SubElement(head, "urlPublic").text = (
      "https://gitgud.io/sjehuda/slixfeed")
  time_stamp = dt.current_time()
  ETR.SubElement(head, "dateCreated").text = time_stamp
  ETR.SubElement(head, "dateModified").text = time_stamp
  body = ETR.SubElement(root, "body")
  for result in results:
      outline = ETR.SubElement(body, "outline")
      outline.set("text", result[1])
      outline.set("xmlUrl", result[2])
      # outline.set("type", result[2])
  tree = ETR.ElementTree(root)
  tree.write(filename)

  https://gitgud.io/sjehuda/slixfeed/-/raw/master/slixfeed/action.py?ref_type=heads